This invention relates generally to actuators and, more particularly, to an actuator which provides a predetermined time delay before automatically resetting a controlled member back to an initial position from which it has been previously displaced by the actuator.
It is a common goal in the design, for example, of high performance gas turbine engines to provide for optimization of the performance of axial flow compressors therein under various operating conditions. One method of achieving this end is to provide compressors with a series of variable angle stator or guide vanes. During normal engine operation, the angle of these vanes is adjusted according to a predetermined schedule, in accordance with speed, thrust, or other engine operating parameters, to optimize the compressor performance.
In addition to optimizing compressor performance, the opening of the variable angle stator vanes may be utilized to control various engine operating parameters such as turbine inlet temperature downstream of the compressor, exhaust gas temperature, and fan speed. More specifically, and for example, it is generally desirable to control the angular orientation of the variable angle stator vanes during engine acceleration and thrust output buildup. The normal manner of increasing thrust output of a gas turbine engine is to increase the amount of fuel delivered to the combustion system. The high fuel flow required to accelerate the engine results in an increase in combustion burner pressure. This, in turn, decreases the air flow at the rear of the compressor and can result in a choking-stall condition of the stator vanes whereby air flow thereover becomes turbulent resulting in reduced air flow and a reduction in compression ratio and, accordingly, a loss in engine performance.
In order to avoid the above-mentioned adverse condition, the angular orientation of the stator vanes can be adjusted. If the angular orientation is not adjusted, upon reaching a desired engine speed the temperature of the gas entering the turbine can exceed the turbine design limit and can result in an appreciable decrease in turbine engine life. This turbine overtemperature problem can be minimized by adjusting the angular orientation for reducing or closing the variable angle stator vanes. The turbine overtemperature, however, normally lasts for only a short period of time, e.g., approximately 30 seconds. It is desirable, therefore, to provide an actuator which can adjust the stator vanes for this short period of time and thereafter automatically reschedule the position of the vanes to their normal positions.
One actuator effective for scheduling the angular position of stator and guide vanes is disclosed in U.S. Pat. No. 3,628,329--Spencer, assigned to the present assignee. This patent, incorporated herein by reference, discloses an engine-temperature responsive actuator effective for positioning a controlled member to generate a correction signal for adjusting the variable angle stator vanes during engine temperature transient conditions, which correction signal becomes substantially zero upon return of the actuator to a null condition which occurs upon engine temperature stabilization.
The actuator comprises a solid rod member positioned coaxially within and spaced from a perforated sleeve member and fixedly connected to each other at corresponding ends thereof. In operation, compressor interstage hot air is directed into contact with and heats the rod and sleeve members causing both members to expand thermally at different rates. During transient changes of compressor interstage air temperature, a relative differential thermal expension exists between the rod and sleeve. This difference in expansion is utilized for positioning the controlled member for predeterminedly positioning the variable stator vanes. After elapse of a finite time period, the temperatures of the rod and sleeve members become equal and the differential expansion becomes substantially zero which thereby automatically reschedules the position of the vanes to a normal position.
The actuator of Spencer can also be used to compensate for engine thermal transient characteristics which, if not provided for, can result in fan speed overshoot or undershoot and exhaust gas temperature overshoot after a new fan speed has been set at the engine control panel. More specifically, during operation of the gas turbine engine, the throttle is set to a desired fan speed from a lower fan speed setting, such as at idle, and the engine is then caused to accelerate. Due to inherent engine thermal characteristics, the actual fan speed, for example, can initially overshoot the desired speed setting and after a relatively short time period also undershoot the desired speed setting. These overshoot and undershoot speed errors can result in a decrease in engine life due to engine overtemperature therefrom and can prevent the thrust output of the engine from meeting the expected output at a given throttle setting. The latter consideration can be important where the engine powers an aircraft and a take-off thrust output must be accurately maintained. The actuator of Spencer is effective to compensate for these speed errors; and the present invention represents an improvement thereover by providing for enhanced time response and compensation for speed or temperature errors, especially initial overshoot errors.